The enzyme CTP:phosphocholine cytidylyltransferase (CT) is a critical member of the CDP-choline pathway, for the biosynthesis of phosphatidylcholine, the major component of eukaryotic cell membranes and a precursor to vital components of signal transduction pathways such as diacylglycerol and phosphatidic acid. CT is rate-limiting for the CDP-choline pathway and therefore is extensively regulated at the cellular level. CT is present as both a soluble and membrane-associated form. In many cells, activation of CT occurs simultaneously with the translocation of the enzyme from a soluble form to membrane-associated form, while in vitro the soluble form of CT is activated by the addition of certain lipids. The proposed research will utilize recombinant forms of CT from rat liver, S. cerevisiae, and C. elegans to investigate the mechanism whereby lipids activate the enzyme. Lipid-binding regions of S. cerevisiae and C. elegans CT will be identified and their solution structures will be solved alongside the rat lipid binding region utilizing Nuclear Magnetic Resonance. Mutant enzymes truncated to remove the lipid binding region of each form of CT will be analyzed both in vitro and in vivo to determine if the lipid binding region of CT serves as an inhibitory element to limit the activity of the enzyme in the absence of activating lipids. Elucidation of the mechanism whereby lipids activate CT will enhance the understanding of the cellular regulation of this pivotal enzyme, which provides the major constituent of mammalian cell membranes, the region of the cell central to the initiation of signal transduction pathways leading to either cell growth or cell death.